1. Field of the Invention
This invention relates to a framework structure of an automobile body formed of a lightweight metal material.
The present application is based on Japanese Patent Application No. Hei. 10-10288, which is incorporated herein by reference.
2. Description of the Related Art
For a lightweight metal material such as aluminum, a long size material which is complicated in cross section can be formed comparatively easily by extrusion or the light weight metal material can be provided with die casting. (Moreover, a strength equivalent to that of steel.) Thus, in recent years, attention has been focused on applications of light weight metal material as a frame material of an automobile.
By the way, a die casting has high rigidity, but is poor in toughness. Thus, if an extrusion is simply welded to the die casting, the die casting is broken due to impact load at the vehicle collision time and impact energy absorption characteristic at the deformation time, one of excellent features of the aluminum material, is impaired.